Re-agree - Indenture of Apostles

One day Sa, a person who refuses to believe in fantasy, sees a pretty girl floating down from the sky. His peace is quickly shattered and he is brought into the world of "Christ" and "Apostles". However, the twist is these "Apostles" are fighting for their lives (and they're girls, not to mention). Who are these "Apostles" and what are they fighting for?